On the diffusion length and grain size homogeneity requirements for efficient thin-film polycrystalline silicon solar cells

Abstract

We examine the influence of intragrain defects and grain boundaries on the macroscopic performance of a thin film polycrystalline silicon solar cell. In addition, we evaluate the effect of grain size inhomogeneity on the cell performance via circuit simulations. From an analytical study of charge transport in individual grains and homogeneous grain systems, we obtain the grain size and intragrain diffusion length requirements for a desired efficiency. We identify the conditions under which the grain size and the intragrain diffusion length dominate the cell characteristics. In devices with intragrain effective diffusion length Lmono ⩽ 100 µm and grain boundary recombination velocity SGB ⩽ 104 cm s−1, achieving a larger grain size beyond several µm is not crucial. The inhomogeneous distribution circuit simulations show that grain size inhomogeneity is not the main limiting factor in polycrystalline silicon solar cells. This is so even in thin polycrystalline silicon films with a broad grain size distribution such as those made with aluminum-induced crystallization at low annealing temperature. The main reason is that the optimum bias point for grains of different sizes only differ by about ∼50 mV over a fairly wide grain diameter range 0.5–50 µm even when Lmono = 100 µm and SGB = 105 cm s−1.